1. Field of the Invention
The present invention relates to a gas-liquid contacting apparatus of a column system for bringing a gas into contact with a liquid. Furthermore, the present invention relates to a process for producing ozone water and a process for oxidizing an organic substance with ozone, each using the apparatus. Moreover, the present invention relates to a process for removing a gas dissolved in a liquid using the apparatus. Also, the present invention relates to a process for treating a volatile substance in a liquid using the apparatus.
2. Discussion of the Background
Known gas-liquid contacting apparatus include spray columns, wetted wall columns, packed columns, and plate columns (hereinafter referred to as xe2x80x9ccolumn gas-liquid contacting apparatusxe2x80x9d), and further include ejectors, bubbling vessels, and static mixers (hereinafter referred to as xe2x80x9cbubbling gas-liquid contacting apparatusxe2x80x9d). The packed columns are representative. Conventionally, the packed columns in most cases have been used for treating gases and, hence, many of these have been used for the purpose of gas cleaning. Typical examples of the gas cleaning are air pollution control processes represented by flue gas desulfurization. In such cases, an important factor in design is how to treat a large quantity of a gas to be treated. However, since too large a gas amount results in a phenomenon in which the liquid is prevented from descending, i.e., the so-called flooding, and in an increased blower operating cost, an appropriate gas load is selected. Generally, employed values of this gas load are from 1 to 2 m/sec in terms of superficial gas velocity u obtained through a density correction.
Generally, a gas velocity U for a packed column is the velocity of a gas which is passing through the vacant column, and has no relation with the voidage which is the volume calculated by subtracting the volume occupied by the packing.
Various gases which are different in density (xcfx81) exist, ranging from light gases, such as hydrogen (molecular weight=2), and intermediate gases, such as air (molecular weight=29), to heavy gases, such as chlorine (molecular weight=71) Even air and steam, which are the most common gases for packed columns, each has considerably different densities depending on pressure and temperature Even when such various gases having different densities have the same gas velocity U, they differ in kinetic energy.
Consequently, the velocities of gases having the same kinetic energy can be expressed in terms of u which is the gas velocity corrected with density and is defined by the following equation (1), and gases having the same value of u have the same kinetic energy regardless of their densities:                     u        =                  U          ⁢                                                    ρ                ⁢                                  xe2x80x83                                ⁢                                  (                                      kg                    ⁢                                          /                                        ⁢                                          m                      3                                                        )                                                            1.2                ⁢                                  xe2x80x83                                ⁢                                  (                                      kg                    ⁢                                          /                                        ⁢                                          m                      3                                                        )                                                              ⁢                      xe2x80x83                    ⁢                      (                          m              ⁢                              /                            ⁢              sec                        )                                              (        1        )            
In equation (1), the numeral 1.2 indicates the density of air at normal temperature and pressure, i.e., 1.2 kg/m3. Using this numeral to convert the densities of gases having various densities into dimensionless numbers, velocities of these gases each can be expressed in terms of the velocity of air at normal temperature and pressure.
In the conventional column gas-liquid contacting apparatus described above, the gas velocities employed have been in the range of from 1 to 2 m/sec in terms of the density-corrected superficial gas velocity u. These apparatus are unsuitable for operations where the gas amount is smaller than the lower limit. Specifically, gas velocities in the range of 0.1 less than u less than 1 m/sec are undesirable from the standpoint of profitability, while gas velocities of u less than 0.1 m/sec are undesirable from the standpoint of performance.
Recently, ozone has been utilized in the various fields instead of chlorine etc. which had been used. Ozone water is used for cleaning of silicon wafers and liquid crystals in electronic industry, sterilization of production apparatus and instruments in the filed of foods, sterilization of vegetables, medical instruments, swimming pool water, and other applications. Also, gas-liquid contacting reactions with ozone gas is applicable to the oxidative decomposition of organic substances in water, oxidative decomposition of COD substances in waste water, decoloring of colored waste water, bleaching of pulps in the pulp industry, and the like. Furthermore, ozone is often used as ozone water in which ozone is dissolved in water as well as gaseous ozone.
Bubbling vessels, ejectors, packed columns and the like are known as ordinary equipment for producing ozone water. However, the efficiency of utilizing ozone is low, and the concentration of ozone water is limited.
Furthermore, the presence of dissolved oxygen in water to be fed to a boiler or in cooling water causes corrosion in the apparatus, pipings, and the like. Accordingly, techniques for chemically or physically removing the oxygen dissolved in the water have been used from long ago.
Also, with respect to cleaning water for use in semiconductor production, attempts are being made to diminish the oxygen dissolved therein due to that the dissolved oxygen produces adverse influences.
Known techniques for removing dissolved oxygen from water include addition of a reducing agent, a contacting with hydrogen in the presence of a catalyst, vacuum degassing under heating, nitrogen gas bubbling, removal with a separatory membrane, and the like.
However, according to the conventional removing processes, a large amount of a dispersion gas is required, and a complicated equipment is required for carrying out the reaction in vacuo under heating. Thus, an effective and simple apparatus using a packed column is not known.
Stripping columns according to a packed bed system or a plate system are conventionally known as means for removing a volatile substance from waste water by stripping. The conventional packed bed stripping columns necessitate a sufficient linear gas velocity for the countercurrent contact of a liquid to be treated with a purge gas and necessitate an exceedingly large stripping gas amount for the stripping of a highly volatile substance.
Therefore, the concentration of the volatile substance in the gas phase is insufficient as compared with that in the liquid phase. The resultant stripping gas containing the volatile substance is treated either with an adsorbent, such as activated carbon or the like, to recover the volatile substance or by incineration. In the latter case, the stripping gas should be burned in a large quantity. Consequently, this technique is economically disadvantageous.
Moreover, since discharged gases have no regulations concerning such volatile substances unlike waste water, there have frequently been cases where waste water is treated by mere exposure to air (air stripping) to discharge the stripped volatile substances as they are into the air.
Recently, there is a fear concerning influences on the human body of volatile substances or endocrine disrupters represented by benzene and toluene, volatile organic halogen substances represented by dichloromethane and chloroform, and analogues thereof. Despite these circumstances, no process has been obtained which is effective in recovering such substances from waste water. Consequently, for removing such substances from waste water, processes necessitating a large amount of energy have been used, such as the process comprising stripping with a large amount of air and recovery by adsorption and the process comprising stripping and decomposition by incineration. Alternatively, a process comprising subjecting waste water to air stripping and discharging the stripped volatile substance into the air has been used due to that the emission of such substances is not regulated by the law relating to air pollution control.
An object of the present invention is to provide an apparatus in which gas-liquid contacting can be efficiently carried out at a low gas feed rate.
Another object of the present invention is to provide an apparatus which is intended not only to be a basic apparatus usable in gas-liquid contacting operations in various applications, but also to have a simple structure.
Also, an object of the present invention is to provide a process for producing ozone water, a process for oxidizing an organic substance with ozone in a liquid, and a simple apparatus used therefor.
Furthermore, an object of the invention is to provide a novel process for removing a gas dissolved in a liquid using a packed column unlike conventional degassing processes, and a simple apparatus used therefor.
Moreover, an object of the invention is to provide a process by which a gas dissolved in a liquid is removed and, simultaneously therewith, a gas different from the removed dissolved gas can be dissolved in the liquid at a high concentration, and a simple apparatus used therefor.
Still moreover, an object of the present invention is to provide an effective and economical process of treatment for stripping highly volatile substances, such as benzene, toluene, dichloromethane, chloroform, and the like, among many organic substances, from liquids to remove or recover the substances, and a simple apparatus used therefor.
Still furthermore, an object of the present invention is to provide a process by which even a liquid containing a volatile substance at a low concentration can be efficiently stripped to concentrate the volatile substance, and a simple apparatus used therefor.
The present invention relates to the following (1) to (15):
(1) A gas-liquid contacting apparatus of a column system, comprising
a column having an inlet for introducing a liquid to be treated and an outlet for a discharged gas in an upper part thereof, and an inlet for introducing a gas and an outlet for a treated liquid after gas-liquid contact in a lower part thereof, and
at least two units comprising a packing material or a wetted wall structure, said units being separated by a gas back mixing preventer plate between the upper and lower parts,
wherein the liquid introduced and the gas introduced are brought into countercurrent contact while keeping the gas phase as a continuous phase.
(2) The apparatus according to the above (1), wherein
the gas back mixing preventer plates are installed at an interval H of 2 m or less,
the ratio H/D is 5 or less, wherein D represents a diameter of the column, and
the gas back mixing preventer plates have a gas passage hole in an amount of 5% or less, in terms of the percentage of the total area of all gas passage holes, to the cross-sectional area of the column.
(3) The apparatus according to the above (1), wherein the unit comprises a packing material (hereinafter referred to as a xe2x80x9cpacked bedxe2x80x9d).
(4) The apparatus according to the above (1), wherein
the gas back mixing preventer plate is a plate comprising gas passage holes and liquid passage holes, and
the packing material is supported by the gas back mixing preventer plate.
(5) The apparatus according to the above (1), wherein
the gas back mixing preventer plate is a plate comprising gas passage holes and liquid passage holes, and
the packing material is supported by a support other than the gas back mixing preventer plate.
(6) A process for producing an ozone liquid, comprising
introducing a liquid into an upper part of a column comprising at least two units comprising a packing material or a wetted wall structure, said units being separated by a gas back mixing preventer plate,
introducing an ozone gas into a lower part of the column, and
bringing the liquid into countercurrent contact with the ozone gas while keeping the gas phase as a continuous phase.
(7) The process according to the above (6), wherein the liquid is pure water or ultra pure water.
(8) A process for oxidizing an organic substance in a liquid, comprising
introducing a liquid to be treated into an upper part of a column comprising at least two units comprising a packing material or a wetted wall structure, said units being separated by a gas back mixing preventer plate,
introducing an ozone gas into a lower part of the column, and
bringing the liquid into countercurrent contact with the ozone gas while keeping the gas phase as a continuous phase.
(9) A process for removing a gas dissolved in a liquid, comprising
introducing a liquid to be treated into an upper part of a column comprising at least two units comprising a packing material or a wetted wall structure, said units being separated by a gas back mixing preventer plate,
introducing a purge gas into a lower part of the column, and
bringing the liquid into countercurrent contact with the purge gas while keeping the gas phase as a continuous phase to remove a gas dissolved in the liquid.
(10) The process according to the above (9), wherein
the gas dissolved is oxygen, and
the purge gas is nitrogen and/or carbon dioxide.
(11) A process for removing a gas dissolved in a liquid, comprising
introducing a liquid to be treated into an upper part of a column comprising at least two units comprising a packing material or a wetted wall structure, said units being separated by a gas back mixing preventer plate,
introducing a purge gas into a lower part of the column, and
bringing the liquid into countercurrent contact with the purge gas while keeping the gas phase as a continuous phase to remove a gas dissolved in the liquid and dissolve the purge gas in the liquid.
(12) The process according to the above (11), wherein
the gas dissolved in the liquid is oxygen, and
the purge gas is carbon dioxide or a combination of carbon dioxide and nitrogen.
(13) A process for treating a volatile substance in a liquid, comprising
introducing a liquid to be treated into an upper part of a column comprising at least two units comprising a packing material or a wetted wall structure, said units being separated by a gas back mixing preventer plate,
introducing a purge gas into a lower part of the column, and
bringing the liquid into countercurrent contact with the purge gas while keeping the gas phase as a continuous phase to strip the volatile substance in the liquid.
(14) The process according to the above (13), wherein the volatile substance is a volatile organic substance.
(15) The process according to the above (13), further comprising
cooling and condensing a stripping vapor containing the stripped volatile substance, and
concentrating and recovering the volatile substance,
wherein the purge gas is water vapor.